In this project we determine the antigenic relationships based on VP4 and VP7 neutralization specificities of various rotavirus strains derived from humans and animals. The elucidation of the neutralization specificities of rotaviruses is important in order to achieve a more comprehensive understanding of rotavirus epidemiology and for formulation of an effective strategy for vaccination. Of 5 globally important VP7 (G) serotypes (G1-4 and 9) of rotaviruses, G9 continues to attract considerable attention because of its unique natural history. Serotype G9 rotavirus was isolated from a child with diarrhea first in the US in 1983 and subsequently in Japan in 1985. Curiously, soon after their detection, G9 rotaviruses were not detected for about a decade in both countries and then reemerged in both countries in the mid-1990s. Unexpectedly, however, such reemerged G9 strains were distinct genetically and molecularly from those isolated in the 1980s. Thus, the origin of the reemerged G9 viruses remains an enigma. Sequence analysis has demonstrated that the G9 rotavirus VP7 gene belongs to one of at least three phylogenetic lineages: lineage 1 (strains isolated in the 1980s in the US and Japan), lineage 2 (strains first isolated in 1986 and exclusively in India thus far) and lineage 3 (strains that emerged/reemerged in the mid-1990s). Currently, lineage 3 G9 viruses are the most frequently detected G9 strains globally. During rotavirus strain surveillance of archival diarrheal stool samples collected at Children's Hospital in Washington DC from 1974 to 1981, we detected 2 G9 viruses in samples collected in 1980. We successfully grew these G9 viruses in cell cultures and then characterized them biologically and molecularly. These viruses were shown by neutralization to be closely related to lineage 3 viruses. However, the VP7 gene of these G9 viruses was found to be placed between lineage 1 and 3 viruses phylogenetically. These two viruses, which are the earliest G9 viruses isolated in humans thus far, may represent ancestral viruses from which lineage 1 and lineage 3 G9 viruses evolved.